this study, PE, PTFE, PS and PMMA were exposed to a He+O2, APGD and pre and post treatment surface chemistries were analyzed by X-ray photoelectron spectroscopy and contact angle measurements. Semi-empirical and ab-initio calculations were performed to correlate the experimental results with sonic plausible molecular and electronic structure features of the oxidation process. For the PE and PS, significant surface oxidation showing C-O, C=O, and O-C=O bonding, and a decrease in the surface contact angles was observed. For the PTFE and PM MA, little change in the surface composition was observed. The molecular modeling calculations were performed on single and multiple oligomers and showed regardless of oxidation mechanism, e.g. -OH, =O or a combination thereof, experimentally observed levels of surface oxidation were unlikely to lead to a significant change in the electronic structure of PE and PS, and that the increased hydrophilic properties are the primary reason for the observed changes in its electrostatic behavior. Calculations for PTFE and PMMA argue strongly against significant oxidation of those materials, as confirmed by the XPS results

Boucher, D.

Calle, C. I.

Trigwell, S.

NASA Technical Reports Server

-'2'-
Electrostatic Properties of Polymers Subjected to 
Atmospheric Pressure Plasma Treatment; Correlation of 
Experimental Results with Atomistic Modeling 
S Trigwell,' D Boucher 2 and C I CaIIe3 
'Electrostatics and Surface Physics Laboratory, ASRC Aerospace, ASRC-24, 
Kennedy Space Center, FL 32899 USA 
2 Department of Physics and Chemistry, King's College, Wilkes-Bane, PA 18711 
USA 
Electrostatics and Surface Physics Laboratory, KT-E, NASA Kennedy Space Center, 
FL 32899 USA 
E-mail: steve.trigweI1-1ksc.nasa.gov 
Abstract. In this study, PE, PTFE, PS and PMMA were exposed to a He+0-, APGD and pre 
and post treatment surface chemistries were analyzed by X-ray photoelectron spectroscopy and 
contact angle measurements. Semi-empirical and ab-initio calculations were performed to 
correlate the experimental results with sonic plausible molecular and electronic structure 
features of the oxidation process. For the PE and PS, significant surface oxidation showing C-
0, C=0, and 0-C=0 bonding, and a decrease in the surface contact angles was observed. For 
the PTFE and PM MA, little change in the surface composition was observed. The molecular 
modeling calculations were performed on single and multiple oligomers and showed regardless 
of oxidation mechanism, e.g. —OH, =0 or a combination thereof, experimentally observed 
levels of surface oxidation were unlikely to lead to a significant change in the electronic 
structure of PE and PS, and that the increased hydrophilic properties are the primary reason for 
the observed changes in its electrostatic behavior. Calculations for PTFE and PMMA argue 
strongly against significant oxidation of those materials, as confirmed by the XPS results. 
1. Introduction 
Art atmospheric pressure glow discharge (APGD) plasma using He+0 2 was used at Kennedy Space 
Center on polymeric spaceport materials to enhance the surface charge dissipation and prevent 
possible electrostatic discharge in spaceport operations [I]. A significant decrease in the charge decay 
after tribocharging the materials with polytetrafluoroethylene (PTFE) felt was observed. Yanagida el 
a/. [2] showed that the threshold for photoemission and ionization potentials of certain polymers is 
related to the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital 
(LLIMO) levels, and in substituted polymers, it was found that the triboelectric charging efficiency 
could be related to the properties of the substitution groups. Their results iiidicated that electronic 
states are responsible for contact electrification in that charge transfer could only occur into a narrow 
window of bulk and surface states close to the Fermi energy within an insulator, and that these states 
were due to molecular ion side groups. Gibson and Bailey [3] showed tribocharging is linearly 
correlated with substituent constants for polyethylene and polypropylene.
https://ntrs.nasa.gov/search.jsp?R=20130011581 2019-08-31T00:41:11+00:00Z
In this study, in order to understand the oxidation process involved in APGD applications to 
polymers and how it affects electrostatic properties, coupons of polyethylene (PE), 
polytetrafluoroethylene (PTFE), polystyrene (PS), and polymethylmethylacrylate (PMMA) were 
exposed to a He+0 2 APGD plasma, and the pre and post treatment surface chemistry was analyzed by 
X-ray photoelectron spectroscopy (XPS) and contact angle measurements. Semi-empirical PM3 
Flatree-Fock and ab-initio DFT molecular modeling calculations were performed to correlate the 
experimental results with plausible molecular and electronic structure implications for different 
arrangements of carbonyl, =0. and hydroxyl, -OH, groups attached to the central carbon chain in both 
polymers. 
2. Experimental 
Coupons measuring 25 x 25 x 2 mm thick were cut out of flat sheets of low molecular weight PE, 
PTFE, PS, and PMMA. The coupons of each material were ultrasonically cleaned for 10 minutes 
successively in acetone, methanol, and iso-propanol, and finally rinsed in DI water. The coupons were 
allowed to air dry in a positive pressure sterile hood to avoid any contamination until used. The He+O, 
plasma (98% He, 2% 02) was applied to each coupon at a power of 150 W. The coupons were exposed 
to the plasma via a 150 x 3 mm slit in the plasma head at a distance of 4 mm. The head vvas scanned at 
a speed of 15 mm/sec-i across the coupons so that total exposure time was 5 minutes. Previous 
experimentation had shown that this was the optimum exposure time as monitored by the 0:C ratio 
measured by XPS [1]. 
XPS analysis of the pre and post APGD treated coupons were conducted on a Kratos XSAM 800 
Spectrometer at a background pressure of 1 x 10-9 torr, using a Mg Ku (h1i = 1253.6 eV) x-ray source. 
The x-ray beam used was 150W, 4 - 6 mm in diameter. The collected data were referenced to the 
carbon Is (Cis) peak at 284.6 +7- 0.5 eV. Wide survey scans were collected from 0- 1100eV at a pass 
energy of 80 eV in I eV steps with a 50 ms dwell time to determine overall elemental composition. 
Narrow scans of the Cis peak were collected at a pass energy of2O eV in 0.1 eV steps with a 300 ms 
dwell time to determine the carbon bonding. The relative atomic concentrations of the detected 
elements were calculated and normalized to 100% using sensitivity factors supplied by the instrument 
manufacturer from known certified standards. The peak curve fitting was performed using XPS 
International SDP v.4.1 data reducing software. 
Contact angle measurements were performed on a VCA Optima XE contact angle instrument (AST 
Products Inc.) enclosed in an environmental chamber. The measurements of the advancing angle were 
performed by depositing 5 pA drops of de-ionized water (18.2 MQ) using a motorized syringe 
assembly, and were taken at 45 +7- 3% RH and at 72 °F. Ten measurements were taken for each 
sample. 
Various models of the plasma-induced oxidation of PB, PTFE, PS, and PMMA were simulated 
using a variety of molecules (singles chains up to 40 carbons in length and even coupled chains) and 
theoretical methods (semi-empirical PM3 Hartree-Fock, followed up by more rigorous ctb-iniiio DFT 
calculations). The calculations were performed using Spartan 02 for Linux/Unix and Spartan 04 for 
Windows. The primary purpose of the calculations was to determine the energetic and electronic 
structure implications for different arrangements of carbonyl, =0, and hydroxyl, -OH, groups attached 
to the central carbon chain in all polymers. The DFT calculations were run on a 3GHz Pentium-l\' 
Linux workstation. 
3. Results and Discussion 
The XPS widescan data for the PE before and after APOD treatment are shown in Figures 1 and 2. 
The high resolution scans of the Cis peak before and after APGD treatment showing peaks at 284.6. 
286.5, 288, and 290 eV corresponding to C-C/C-H, C-0, C=O, and O-C=0 bonding, respectively, are 
shown in Figures 3 and 4. Only the XPS scaiis for PE are shown for brevity. The XPS data is 
summarized in Tables 1 and 2.
Counts 
340O0_	
Clo 
(a) 
00009-
200001 
22000k 
t80O9 CUOLLI 
10000	 01 
800 
200
1000	 800	 600	 400	 200	 0 
OtnOtnO Enerly. (e'O 
Figure 1. XPS spectrum of FE as-received. 
0)0	 (a)	 C-dC-H 
oo	 - 
4001 
80CC 
04)0 - 
10001__	 I 
283	 260	 200	 244	 201
01114108 6oCt88, (e03
Z200C
Clo 20000 (b) 
8000 
	
16000	 CIKLLI 
14000 OIKLLJ 
17000 
10000 
8000 
6000 
4000 
2000 
	
-	 1000	 000	 600	 400	 200	 0 
BindIng Energy, )eOO 
Figure 2. XPS spectrum of PE after APGD. 
COur4S
(b)	
.	 C-C/C-Il 
(: 
0.CnuOC() 
.	
/ 
Figure 3. XPS spectrum of PE Cis peak as- 	 Figure 4. XPS spectrum of PE Cis peak after 
received.	 APGD. 
Table 1. Relative atomic concentrations of the polymer surfaces before and after He+0 2
 APGD
treatment. 
C 0 F	 Si	 N 
(at. %) (at. %) (at. 0/)	 (at. 0/)	 (at. 9'o) 
FE 98.7 1.3 -	 -	 - 
PE after APGD 72.3 27.7 -	 -	 - 
PTFE 34.6 0.5 64.9	 -	 - 
PTFE after APGD 32.0 2.1 65.4	 0.5	 - 
PS 89.7 10.3 -	 -	 - 
PS after APGD 73.2 26.3 -	 -	 0.5 
PMMA 68.4 31.6 -	 -	 - 
PMMA after APGD 67.6 32.4 -	 -	 -
An increase in the oxygen Is (Ols) peak indicating increased oxidation of the surface is clearly 
observed for PE and PS. Very little difference was observed in the surface composition of the PTFE 
and PMMA as a result of the APGD treatment although a slight increase in the oxygen concentration 
was observed. Minor contaminants were detected on the surface of the PTFE and PS after the 
treatment. From the Cis peak scans in Table 2, significant oxidation in terms of C-O, C=0, and 0-
C=0 were now observed for the PE, while for the PS, increases in the C0 and formation of O-C=O 
bonds were observed. However, very little difference was observed in the Cis chemistry for both the 
PTFE and PMMA before and after APGD treatment. 
Table 2. CIs peak functional group atomic concentrations from the XPS data before and after l-le+07 
APGD treatment. 
C-C/C-H 
(at. %)
C-O 
(at. %)
C=O 
(at. %)
O-C=O 
(at. %)
C-F 
(at. 0/)) 
PE 100.0 - - - - 
PB after APGD 64.1 14.4 7.7 13.8 - 
PTFE 10.2 1.4 - - 82.4 
PTFE after APGD 7.2 1.4 - - 86.9 
PS 64.4 30.8 4.8 - - 
PS after APGD 55.1 30.7 8.5 5.7 - 
PMMA 58.2 21.3 1.3 19.2 - 
PMMA after APGD 52.9 22.3 2.9 21.8 - 
The contact angle data is presented in Table 3. The mean contact angle for the PE decreased from 
98.9° to 61.2° after APGD indicating increased hydrophilicity, but for PS only changed from 80.9° to 
77.6°. For the PTFE, no oxidation was detected but the surface contact angle decreased from 1 22.4° to 
108.4° after APOD, although still within the hydroph2bic range (> 90°). The PMMA contact angle 
slightly increased but was similar to as before treatment. For the PE, the significant increase of the 
oxygen surface is consistent with what has been observed for Ultra-high-weight PE fibers exposed to a 
I-Ic low pressure plasma [4]. For PS, it has been reported that the oxidation of the aromatic rings 
occurs [5-6], but it was not possible to tell in our data. Wells et al. [7] obser'ed that PS experiences a 
greater oxidation than other polymers that can be attributed to the unsaturated/chromophobic phenyl 
groups in PS being highly vulnerable towards plasma activation. Oxidation of PTFE by APGD in air 
has been reported, but only for exposure times of 20 to 120 minutes [8], while long exposure times to 
an oxygen plasma (> 15 mins.) showed there was some initial oxidation, but the PTFE was chemically 
similar to the control [9]. 
Table 3. Contact angle data before and after He+0 2 APGD treatment. 
Mean Standard 
(degrees) Deviation 
FE 98.89 2.97 
PEafterAPGD 61.22 5.03 
PTFE 122.35 7.39 
PTFE after APGD 108.43 5.74 
PS 80.96 4.76 
PS after APGD 77.61 3.93 
PMMA 69.07 4.68 
PMMA after APGD 72.42 3.59
The difference between the work functions of two materials is a rough guide to the level and sign 
of charge transfer during triboelectrification. To estimate the work function, p, of the polymers. it was 
decided to simply use the highest occupied molecular orbital, HOMO, instead of relying upon any 
information from unoccupied states, e.g. the lowest unoccupied molecular orbital, LUMO. So we are 
estimating the work function as the energy to remove an electron from the HOMO level ( p - 
El-IOMO). For large band-gap insulators such as these, the participation of excited states is not likely 
to be important as these states will not be occupied at room temperature and also, the LUMO levels 
are unreliable unless sophisticated configuration interaction (CI) calculations are performed. (The 
LUMO varies wildly for different oxidation scenarios, which can lead to misleading variations in a 
band gap calculated simply by taking the difference between it and the E-TOMO. See Figure 5.) Cl 
calculations were not possible with large chains such as the ones used in this study. For the DFT 
method used (B3LYP with a 631G* basis Set), the HOMO is known to be reliable [10] for relative 
changes or trends, but is not expected to be accurate for absolute ionization energy (see below). 
	
0	 -	 -4 
- - -HvBFllvO
	
- --.'	 —a--- ur FOvO
Figure 5. HOMO and 
	
-10	 -	 --	 -	 - - -. -	 LUMO as a function of 
	
- - - __	
-	 -	 - - -	 -	
and =0 substitutions 
	
-12	 . ............	 forPE 
o	 2	 4	 12	 i6	 fiS	 (6	 (6	 (6 
% o,dion 
Using a 24-carbon PE model, more than 30 different oxidation patterns were simulated with the 
simpler PM3 calculations. Several of these were followed up with DFT calculations, which showed 
the same trends. The HOMO level for PE generally increases (becomes less negative) with increasing 
oxidation, implying a decrease in the work function with oxidation. This change is not universal in that 
for some oxidation patterns the HOMO slightly decreases. However, neither is it dramatic. The energy 
level changes from the pure PE molecule a maximum of+ 0.75 eV (12% oxidation with —OH groups) 
to a minimum of —0.1 eV (66% oxidized with a mixture of —OH and =0 groups). A few models of 
other oxidation scenarios, e.g. ester and ether formation, yield very similar results. This behavior is 
consistent with previously known data on PE (work function = 4.90 eV) and PE oxide (work function 
= 3.95 -4.50 eV) [11]. 
The results of these calculations show the relatively minor variation in the HOMO level as 
oxidation is increased dramatically, even well beyond levels seen in the experiments. This suggests. 
therefore, that the major factor contributing to the decrease in the triboelectric behavior of PE upon 
plasma treatment is its increased hydrophilic behavior and not a dramatic change in its intrinsic 
electronic structure. We consider this a key point of the paper. 
The c/lange in the work function is modeled well by the calculations, though clearly the absolute 
values differ from experimental values. Better modeling of excited states or ionized species (once an 
electron is removed) or more sophisticated modeling of PE, using actual slabs rather than isolated 
oligomers, may improve absolute agreement. Simulation of some pairs of smaller PE oligomers (20 
carbons) showed the same general behavior upon oxidation, but faster convergence to n= behavior 
was found than with single, longer oligorners and was regarded as an interesting avenue for future 
model development. 
A similar analysis for PTFE was omitted due to the fact that no oxidation was observed in the 
experiments. To correlate this observation with the atomistic simulations, a series of DFT total energy 
calculations for 0 2 combining with a 24-carbon chain of either PE or PTFE were performed. 
Consistent with this and with chemical intuition, it was found that PE oxidation (either forming C-0l-1 
or CO substitutionals) was favorable, being very exotherrnic, whereas PTFE oxidation (either 
forming C-O-F or C0 substitutionals) was endothermic. The numbers from the DFT calculations 
\'er generally within 25% of those obtained using standard bond enthalpies from the literature.
The chief feature of PS is the presence of the aromatic rings. The carbon 2p states form bonding 
and anti-bonding it clouds, the bonding being the HOMO orbital and the anti-bonding the LUMO of 
the aromatic ring. This feature persists in PS, which in an ideal geometry would have many identical 
rings with identical HOMO and LUMO states. In the actual molecule, and indeed in our theoretical 
modeling, not all rings are equivalent due to slightly different environments caused by the slightly 
differing orientations of the rings around the oligomer, the backbone of which may bend slightly. 
Oxygenating PS introduces C-OH or C0 bonds that are far lower in energy than the electronic states 
of the aromatic rings, a few eV below the HOMO. So, an oxygen atom added to a ring (in the form of 
an —OH) simply disrupts the states on that ring, but the other rings are isolated enough to remain 
unaffected. So, unless all rings are oxygenated, the HOMO, and by analogy the work function, of the 
molecule, is unaffected. Oxygen bonded to the backbone does even less to change the electronic 
structure. We also tested many scenarios in which a ring was opened and oxygenated and saw nothing 
significant in the HOMO/LUMO electronic structure. Our energetic studies indicate that PS readily 
bonds with oxygen, in agreement with our XPS data. 
PMMA does not so readily oxygenate, according to our calculations. There is a slight energetic 
preference for adding more oxygen to this molecule, but not nearly as much as for PS or PE. This is 
consistent with our XPS data which shows little additional oxygen upon APGD He+O, treatment. As 
far as the electronic structure is concerned, oxygenation changes almost nothing about the PMMA 
molecule, which already has C—Ol-1 and C=O bonds in abundance. 
4. Conclusions 
Atmospheric pressure glow discharge plasma significantly improved the hydrophilicity of PE as 
determined by the increase in surface oxidation and decrease in the surface contact angle. Although 
increased oxidation was observed for the PS, there was no significant increase in the hydrophilicity. 
However, at this power level, PTFE and PMMA resisted oxidation by the APGD plasma, resulting 
only in minor changes to the surface characteristics. 
The modeling calculations showed relatively minor changes in the HOMO level for increased 
oxidation of PE, indicating that the observed changes in the electrostatic behavior after APGD were 
probably due to the increased surface hydrophilicity rather than significant changes in electronic 
structure. As with PE, we can conclude that any change in the triboelectric properties of PS or PMMA 
upoil APGD treatment is due to changing hydrophilicity and not due to any fundamental change in the 
electronic structure that might make the material more readily charged by electron transfer. 
The calculations for PTFE strongly argued against significant oxidation of the material with 
APGD. as was observed and confirmed by experiment and the XPS results. 
5. References 
I]	 Trigwell S, Schuerger A C, Buhler C R, and Calle C I Proc.37th Lunar and Pla,ieiarv Science 
Coiif March 13-17, 2006. Houston, TX, Paper #2257 
[2j	 Yanagida K, Okada 0, and Oda K 1993 Jpn. J. AppI. Phys. 32 5603-10 
[3] Gibson H W aiid Bailey F C 1977 Chem. Pitys. Letis. 51 2352-55 
[4] Qiu Y, Hwang Y J, Zhang C, Bures B L, and McCord M G 2002 1. Aclhes. Sci. Technol. 16 449-
457 
[5] Grant J L, Dunn D 5, and McClure D J 1988 J. Vac. Sci. Technol. A(6)4 2213 
[6] Shard A G and Badyal J P S 1991 J. Phys. Che,n. 95 9436 
[7] Wells R K, Badyal J P 5, Drummond I W, Robinson K 5, and Street F J 1993 J Adhe,rion Sri. 
Technol. 7 1129 
[8] Fang Z, Qiu Y, and Luo Y 2003 J. Phys. D: App!. Phys. 36 29 80-5. 
[9] Morra M, Occhiello E, and Garbassi F 1990 Suif Inlfce. Anal. 16 4 12-7 
[10] Muchall H M, Werstiuk N H, and Choudhury B 1998 can. I Chem. 76 221-7 
[II] Cross J A 1987 Electrostatics; Principles, Problems and Applications, (Chichester UK: Adam 
H ilger).


Electrostatic Properties of Polymers Subjected to Atmospheric Pressure Plasma Treatment; Correlation of Experimental Results with Atomistic Modeling

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